A number of copolymers of tetrafluoroethylene are known, but new copolymers of tetrafluoroethylene are always of interest due to a desire to obtain polymers having improved properties over polymers known heretofore.
U.S. Pat. No. 4,123,602 to H. Ukihashi, et al., relates to tetrafluoroethylene/ethylene copolymers modified with 0.1 to 10 mol. % R.sub.f CH.dbd.CH.sub.2 where R.sub.f is C.sub.n F.sub.2n+1 in which n is an integer between 2 and 10. These polymers contain between 40 and 60 mol % ethylene. The thermal instability against oxidation limits their use above 150.degree. C. for extended periods of time.
U.S. Pat. No. 3,804,817 to L. A. Wall and D. W. Brown describes copolymers of (perfluoropropyl)ethylene (PFPE) and tetrafluoroethylene (TFE), and copolymers of (perfluoroethyl)ethylene and tetrafluoroethylene (See column 2, lines 16-26. These copolymers are elastomeric, soluble in fluorinated solvents, and possess only modest thermal stability as evidenced by the thermal gravimetric analysis (TGA) data shown in the patent. In J. Poly. Sci. 8, 2441(1970) the patentees reproduce Table 1 of the U.S. Pat. No. 3,804,817 and in the article describe that the copolymers of TFE and PFPE containing up to 89 mol % TFE are amorphous by X-ray diffraction measurement. The polymers appear amorphous on testing by differential scanning calorimetry (DSC) techniques.
Also disclosed in U.S. Pat. No. 3,804,817 is a TFE copolymer containing about 6 mol % PFPE, in Example 4. This copolymer has the following properties:
1. The melting behavior by DSC is essentially identical to PTFE. It is well known (Polymer Handbook, Vol. 2, pV-32, (1975)) that polytetrafluoroethylene (PTFE) has a reproducible melting point at 327.degree. C. and occasionally contains phases which exhibit higher melting points on the initial heating cycle. The higher melting point can be as high as 342.degree. C. The 94/6 copolymer of U.S. Pat. No. 3,804,817 Example 4 exhibits both of these melting points and no lower melting points which would correspond to crystalline copolymer phases.
2. Thermal stability of the melt at 350.degree. C., i.e., approximately 20.degree. C. above the melting point, as measured by isothermal TGA in air, is low. A weight loss of 8.7% was seen at 140 minutes.
3. A small-angle X-ray scattering scan does not show the broad diffraction peak at low angles (2.theta. below 1.degree.) normally seen for crystalline TFE copolymers. The 94/6 copolymer of U.S. Pat. No. 3,804,817 Example 4 instead shows the steady increase in intensity with decreasing angle that is shown by homopolymer PTFE.
Based on these results, the copolymer of U.S. Pat. No. 3,804,817 Example 4 is believed to be composed mostly of a fraction of crystalline TFE homopolymer with a minor fraction of amorphous TFE/PFPE copolymer with high PFPE content. It is well known that crystalline TFE homopolymer cannot be processed by melt techniques, and that mixtures of PTFE with melt processible copolymers lead to the formation of high local concentrations of PTFE, or "fisheyes", in finished parts which are undesirable. In extreme cases, the presence of PTFE in a melt processible copolymer can greatly lower the strengh of a finished article. The objective of copolymerizing small amounts, e.g. 1-10 mol % of comonomer, with TFE is to produce copolymers with homogenous, readily processible melts in which the melting point has been lowered from that of PTFE. U.S. Pat. No. 3,804,817 does not teach how to achieve this objective.